1. Field of the Invention
The present invention relates to the reception of digital television signals transmitted, for example, by a cable network or a satellite. Such television signals require a conversion system to adapt the digital signals to the standardized analog input signals of a television set. The analog input signals of a television set include at least one of signals providing the intensity of each primary color (RGB), or signals providing a composite video signal (CVBS), or separate luminance signals Y and chrominance signals C, the separate luminance and chrominance signals being the signals used for some video tape recorders, particularly S-VHS video tape recorders.
2. Discussion of the Related Art
Digital television signals generally are coded, on the transmit side, according to a data compression process in order to minimize the amount of digital data to be transmitted. This compression is standardized (MPEG). On the receive side, the images are thus generally decompressed in a digital form before being coded according to a color television standard (PAL, NTSC, SECAM) and converted into analog signals. These transformations are generally performed within a conversion system.
FIG. 1 shows, in the form of a block-diagram, an example of a conventional conversion system. Digital television signals transmitted by a satellite reception antenna (not shown) or by a cable network are sent, after a first analog demodulation of the channel frequency and a second demodulation, of the QAM type (quadrature amplitude modulation) for the cable network and of the QPSK type (quadrature phase skip modulation) for the satellite, to an input E of a digital unit 1 of the conversion system. This unit 1 essentially includes a decoder (MPEG-DEC) 2 that receives the digital signals coded according to an MPEG standard and received by input E. Decoder 2 reads out a digital image data flow I.
Unit 1 further generally includes a circuit (OSD-GEN) 3 for generating graphic elements to be inserted in the image data flow. These graphic elements generally correspond to interactive menus, called by means of a remote control, that enable the user to set certain operating parameters of the television set and its receive unit. These can be, for example, settings of the receive channels, settings of the satellite reception dishpan, sound or contrast settings, etc. The signals relative to these graphic elements are generated in a digital form by circuit 3 in a format compatible with that of unit 1, that is, with digital image flow I. These signals are generally called "On screen display" signals, or OSD signals. The OSD signals are inserted in image flow I by means of a mixer (MIX) 4, an output S of which provides a digital image flow resulting from the mixing of the main images I supplied by decoder 2 and the OSD signals. Generator 3 and decoder 2 generally share a same memory (MEM) 5 associated with digital unit 1.
Digital image flow S is then sent to a unit 6 for coding the signals into analog signals which conform with the PAL, SECAM or NTSC color television standards. Unit 6 essentially includes a coding circuit 7 and digital-to-analog converters 8 which transpose flow S into analog CVBS and/or R, G, B and/or Y, C, video signals. The signals provided by unit 6 are used by the television set to display the images.
Some of these signals may also be used to record television programs, for example, on a video tape recorder. When this is the case, the presence of OSD signals inside image flow S is problematic because these OSD signals are then present on all the analog outputs of conversion unit 6 and thus are recorded on the tape of the video tape recorder. Thus, the recording of the television programs is disturbed by any calling of a menu by the user by means of the remote control. Accordingly, it is not possible to check or modify settings during a recording.
To overcome this disadvantage, it is conventionally required to use two digital decoding units and two coding units such as described with respect to FIG. 1 within the conversion system. FIG. 2 shows an example of a conventional conversion system of this type. The conversion system includes two digital units for decoding according to an MPEG standard, respectively units 10 and 20. Each unit 10 or 20 receives a same flow of digital television signals as an input, respectively signals E1 or E2. Units 10 and 20 have substantially the same structure as digital unit 1 of FIG. 1. Thus, they each include an MPEG decoder (MPEG-DEC), respectively 12 or 22, an OSD signal generator (OSD-GEN), respectively 13 or 23, a mixer (MIX), respectively 14 or 24, and a memory (MEM), respectively 15 or 25.
Each unit 10 or 20 is associated with a coding unit, respectively 16 or 26, for converting the digital image flows, respectively S1 and S2, provided by units 10 and 20 into standardized analog video signals. Units 16 and 26 are shown in FIG. 2 by a simple block; each unit 16 and 26 being identical to the unit 6 shown in FIG. 1.
Unit 10 and unit 16 generate analog video signals including, when necessary, OSD signals inserted in the flow I of main images. The operation of units 10 and 16 is identical to the operation of units 1 and 6 described with respect to FIG. 1. Units 20 and 26 generate analog video signals free of OSD signals. Generator 23 of unit 20 is thus unused. Similarly, only outputs CVBS2, Y2, and C2 of unit 26 are used since the outputs R, G, and B of a coding circuit are generally not used for the recording of video images on a video tape recorder.
Two possibilities are thus available for signals CVBS, Y, and C, a first possibility (CVBS1, Y1, C1) including, when necessary, OSD signals for the displaying of interactive menus on the television set, and the second possibility (CVBS2, Y2, and C2) including the flow of main images only. A disadvantage of the conversion system such as shown in FIG. 2 is that it requires four circuits 10, 16, 20, and 26, which are generally integrated. The cost of the system is then substantially doubled with respect to a system such as shown in FIG. 1.